In recent years, there have been dramatic improvements in LED technology such that LEDs of increased luminance and color fidelity have been introduced. Due to these improved LEDs and improved image processing technology, large format, full color LED video screens have become available and are now in common use. LED displays typically comprise a combination of individual LED panels providing image resolution determined by the distance between adjacent pixels or “pixel pitch.”
Outdoor displays that are intended for viewing from great distances have relatively large pixel pitches and usually comprise discrete LED arrays. In the discrete LED arrays, a cluster of individually mounted red, green, and blue LEDs are driven to form what appears to the viewer as a full color pixel. On the other hand, indoor screens, which require shorter pixel pitches such as 3 mm or less, typically comprise panels carrying red, green, and blue LEDs mounted on a single electronic device attached to a driver printed circuit board (PCB) that controls the output of each electronic device.
In order to use an LED chip in conventional applications it is known to enclose an LED chip in a package to provide environmental and/or mechanical protection, color selection, light focusing and the like. An LED package also includes electrical leads, contacts or traces for electrically connecting the LED package to an external circuit. In a typical two-pin LED package/component 10 illustrated in FIG. 1, a single LED chip 12 is mounted on a reflective cup 13 by means of a solder bond or conductive/non-conductive epoxy. One or more wire bonds 11 connect the ohmic contacts of the LED chip 12 to leads 15A and/or 15B, which may be attached to or integral with the reflective cup 13. In FIG. 1, the LED package comprises a vertically oriented LED chip 12 with a conductive growth substrate (p-side up in a group III-nitride LED) or conductive carrier substrate (n-side up) and one wire bond 11. In alternative embodiments, the LED package comprises a laterally oriented LED chip on an insulating substrate with two wire bonds. In certain “flip” chip embodiments, no wire bonds are necessary as would be understood by one of skill in the art. The reflective cup 13 may be filled with a transparent encapsulant material 16 and/or a wavelength conversion material, such as a phosphor, can be included in over the LED chip or in the encapsulant. Light emitted by the LED at a first wavelength may be absorbed by the phosphor, which may responsively emit light at a second wavelength. The entire assembly can then be encapsulated in a clear protective resin 14, which may be molded in the shape of a lens to direct or shape the light emitted from the LED chip 12.
A conventional LED package 20 illustrated in FIG. 2 may be more suited for high power operations which may generate more heat. In the LED package 20, one or more LED chips 22 are mounted onto a ceramic based carrier such as a printed circuit board (PCB) carrier, substrate or submount 23. One or more LED chips 22 may include: a UV, blue or green LED chip, such as a group III nitride based LED chip comprising negatively doped (n-type) epitaxial layer(s) of gallium nitride or its alloys and positively doped (p-type) epitaxial layers of gallium nitride or its alloys surrounding a light emitting active region, a red LED chip, such as an AlInGaP based red LED chip, a white LED chip (e.g., blue LED chip with phosphor(s) layer(s)), and/or a non-white phosphor based LED chip. A metal reflector 24 mounted on the submount 23 surrounds the LED chip(s) 22 and reflects light emitted by the LED chips 22 away from the package 20. The reflector 24 also provides mechanical protection to the LED chips 22. One or more wirebond connections 21 are made between ohmic contacts on the LED chips 22 and electrical traces 25A, 25B on the submount 23. The mounted LED chips 22 are then covered with a transparent encapsulant 26, which may provide environmental and mechanical protection to the chips while also acting as a lens. The metal reflector 24 is typically attached to the carrier by means of a solder or epoxy bond.
Conventional LED packages such as those shown in FIGS. 1 and 2 have transparent encapsulant and transparent reflective cups covering LED chips so that light emitted from LED packages can be used efficiently. Those skilled in the art routinely engineer package components to be light transparent and not to absorb any light generated by the LED or irradiating the package from external sources. When used in LED displays, however, the transparent encapsulant and transparent reflective cups in conventional LED packages can reflect too much background light. When viewing displays including conventional LED packages, customers may have a problem viewing the displayed content if the display reflects too much background light. For example, a customer may find it difficult to read the displayed text under the sun if the display reflects most of the sunlight. Thus, there is a need for a display and LED packages that reflect less background light.
Display customers prefer high contrast displays with low reflection. In addition, customers prefer displays with minimum reflection when the displays are exposed to bright background illumination. Accordingly, a new LED device is provided having improved screen contrast and reduced background light reflection.